Case Studies in Structural Engineering 4 (2015) 2638
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Case Studies in Structural Engineering
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Hand verification for flexural strength of existing R.C. floorssubject to degradation phenomena
G. Campione , F. Cannella, L. CavaleriDICAM, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy
a r t i c l e i n f o
Article history:Received 28 May 2015Received in revised form 14 June 2015Accepted 18 June 2015Available online 24 June 2015
Keywords:T beamR.C. floorArch strength mechanismCorrosionBondPitting
a b s t r a c t
In the present paper, a simplified model for hand verification of the flexural and shearstrength of existing corroded T beams cast in place of lightened R.C. orthotropic slabsforming floors is presented and discussed. Diffused and pitting corrosion on steel bars,compressive concrete strength degradation and concrete bond strength degradation areincluded in the model. The original contribution of the paper is evaluation of the flexural andshear strength considering both the cases of strain compatibility and absence of compatibil-ity and considering the main parameters governing the corrosion process. An arch-resistantmodel for the calculus of the flexural and shear strength of the beam was adopted in theabsence of strain compatibility, while the plane section theory was adopted for the case ofstrain compatibility. No punching shear is considered. This approach is simple and can beapplied on the basis of the experimental information available (carbonation test, chloridecontent, measurement of the pitting in the bar, gravimetric method for general corrosion)or by utilizing analytical expressions calibrated on the knowledge of the corrosion currentintensity determined by linear polarization resistance measurement (LPR). The model wasalso verified against experimental results recently obtained by the authors.
2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/4.0/).
One of the most common types of orthotropic slab utilized in the Mediterranean area to form floors, between 1930 and1970, was the type constituted by low thickness reinforced concrete beams having a T cross-section placed at a distanceof between 200 and 500 mm and lightened with interposition of brick blocks (see Fig. 1). Fig. 1 shows a section of the slabconstruction, as indicated in textbooks of the past 50 years (e.g. ), with the typical arrangement of the steel reinforcements.The slab was constituted with reinforced concrete beams cast in place and having a T cross-section placed at intervals of310 mm. The height of each beam was 170 mm (a value which respects the limit of minimum thickness of 1/30 of the spanadopted in ). Each T beam was formed by a web of 70 mm and a height of 120 mm and an upper flange with minimumthickness 40 mm. Among the T elements brick blocks were placed, 120 mm high, 240 mm wide and 250 mm deep. The T
beams were reinforced with smooth bars in the tension zone with straight or hook anchourages. The minimum percentage ofsteel reinforcement to be placed in the tensile zone was 0.25% of the cross-section area of concrete. The steel reinforcementwas constituted by two bars of small diameter for each T beam. In the first two decades of the century it was commonto adopt one straight bar and one bent one (see Fig. 1). In the second two decades of the century, it became common to
Corresponding author. Tel.: +39 3204395955; fax: +39 091427121.E-mail address: [email protected] (G. Campione).
http://dx.doi.org/10.1016/j.csse.2015.06.0012214-3998/ 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
G. Campione et al. / Case Studies in Structural Engineering 4 (2015) 2638 27
List of symbols
As cross-section area of longitudinal barsbf beam width increased by corrosion crackingb beam section width in the virgin state concrete cover thickness bar diameterred reduced bar diameterfc mean value of concrete compressive strengthfy yield stress of longitudinal bar (or tie)K coefficient related to concretenbars number of bars in one layern number of bars in the supportui corr opening of corrosion crackwcr total crack width for one corrosion levelX corrosion attack depthApit bar cross-section area reduction due to pitting0 strain at peak compressive stressqres residual bond strengthB distance from each T beamH height of each beamL maximum spanlanc anchorage lengths minimum thickness of the upper flangea shear spanj dimensionless internal armPmax maximum value of pit penetration
Pav medium value of pit penetration
dopt two straight bars. The concrete cover thickness was 20 mm. The shaped bar was bent at 1/5 of the span, a lengthorresponding to the zero point of bending moment of a fixed beam subjected to a uniform distributed load.
In the flange of the T beam secondary steel reinforcements having a small diameter (4 or 6 mm) were placed, equivalento 20% of the main reinforcement. No specific shear steel reinforcements were adopted in this type of structure. The shearorce at the supports was provided by the web which, if necessary, was increased in thickness. This increase was madeossible by not placing brick blocks in this zone.
In the first four decades of the century the maximum span adopted for this type of structure was between 3000 and000 mm, while more recently with a thickness of the T beam of 200 mm the span was between 5000 and 6000 mm.
Corrosion phenomena are caused by the loss of the protection provided by the concrete when carbonation occurs. Anotherause of corrosion is the presence of chlorides. Carbonation and corrosion phenomena produce concrete cover spalling andeduction in the steel area. In some cases the damage is not too severe (see Fig. 2a and b) to be visible and needs externalnspection; if more severe damage occurs, it is evident, from Fig. 2c and d and it is also dangerous for the safety of people.n all these cases safety assessment under current conditions is crucial. In some cases it is necessary to design retrofitting oftructures to reproduce the initial safety conditions.
For safety assessment the deterioration and decay of materials must be taken into account . This information cane obtained through destructive testing on materials (visual inspection, depth of carbonation, chloride content, gravimetricests on the bars, measuring the depth of the pit, the corrosion current density on the bar measured experimentally, e.g.sing linear polarization resistance measurement LPR , and non-destructive testing on structures (loading tests)).
For the analytical prevision of safety conditions the first problem that arises is the choice of a suitable calculation model,hich in many cases of severe damage may not be the sectional model because of the loss of bond. The second problem is
he choice of the confidence factor (CF), which depends on the level of detail and extent of the investigations.Another important question not analyzed in the present paper, but of sure interest in future researches, could be the
hoice of the safety factor (or strength reducing factors) and their calibration. To do this the statistical aspect of parametersnfluencing the corrosion processes should be know.
The current research referred to the real cases analyzed by the writers in relation to R.C. buildings existing for the past0 years in the Italian territory and in the province of Palermo. For these structures, it was observed that in the presence
f aggressive environments about 25% of the ceilings of the basement and 35% of the ceilings of the roof of the building areffected by corrosion and carbonation processes. These phenomena are also influenced by the low cover thickness adopted,hich was between 7 and 24 mm.
28 G. Campione et al. / Case Studies in Structural Engineering 4 (2015) 2638
Fig. 1. Constructive section of R.C. floor according to Santarella .
Fig. 2. Degradation phenomena in R.C. floors due to carbonation and chloride attack.
G. Campione et al. / Case Studies in Structural Engineering 4 (2015) 2638 29
Fig. 3. Flexural section model for corroded R.C. beams.
The carbonation depth measured was in many cases between 33 and 45 mm, greater than the thickness of the concreteover and the bars most frequently used, 12 or 14 mm in diameter, reduced to 9 and 11 after 30 years. In the case of theldest constructions one of the two bars is straight up to the support and the other one is bent in the upper portion of theeam (typical reinforcement of a fixed beam). More recently, both bars are present in the support.
The present work examines the problem of corrosion and carbonation in R.C. floors for the past 50 years in the Mediter-anean area which ut
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